I think this might be a bug.
I am exporting a .3gp for iPhone with the 3gp export settings. My original aspect ratio is 16:9. I select .mp4 and 320x240. There is a checkbox that says "preserve aspect ratio", I check it and select "letterbox" expecting black bars at the top and the bottom. When I export it does not preserve aspect ratio and stretches the the mov.
Anyway, my workaround was to open the comp in a new 320x240 comp scaled to 25% with black bars at the top and bottom -- plays fine on an iPhone and behaves exactly as the higher quality .m4v with the 16:9 aspect ratio.
Can anybody else reproduce the "preserve aspect ratio" checkbox not doing a thing in the .3gp export settings? I wasted a lot of time trying to figure this out -- maybe I'll save somebody else some time.
AE CS4 - Mac OSX 10.6.1
No problems here on PC. If it is a bug, then it's either platform specific or dependent on a specific combination of things... Have you tried exporting using H.264 from the render queue? It also can generate 3GP compliant output.
It seems like you're exporting using the File > Export command, which is really not a recommended way to export a video file (and AE displays a warning to tell you so). Video export options in the File > Export menu are generic Quicktime export components, which are not even aware they're being invoked from AE. Because of this, this sort of problem is not suprising at all.
Use the Render Queue instead to export 3GPP video (it's a preset when you pick the H264 format). If you want to letterbox it, nest it in a 4:3 Comp, fit it to Comp Width (Layer > Transform > Fit to Comp Width) and then send that to the Render Queue. The 3GPP preset will instruct you in the comment field to stretch the Comp size in the Output Module to one of the valid 3GPP sizes.